My truck has been making a whirring noise, sorta like a bad bearing, in the rear. I think it's something with the rear diff.

There are two noises.

One is a metallic whirring that I only hear while engine braking. If I downshift, I hear the noise. As soon as I get on the gas or take the clutch out, this noise goes away.

The other is similar, but lower pitched and is most apparent while moving at low speeds. It isn't affected by throttle or clutch.

The noise disappears if I remove the rear driveshaft and drive around on my front wheels.

Does this sound like a problem with my rear differential?

 

Sounds like you got bad u-joints. When engine braking or letting off the throttle on the hwy causes everything to wear on the backside.

Try putting the truck on stands and spin the d-shaft. Mine started to squeak when I let off the throttle on the hwy after a long trip. The rear u-joint was toast!!

IF it is your rear differential, drain the fluid and see if there is any metalic particles in there. Find out what gear ratio you have (if its 4.10 then your on easy street) and perhaps pick up a used 3rd member.

 

I replaced the U-joints while I had the driveshaft out. Noise was still there when I put it back.

I drained and filled the differential a few months ago. The noise was there before and after I did this. I didn't notice anything unusual but I could be wrong.

How do I find out my gear ratio and what can I expect to spend for a used 3rd member?

 

This turned out to be the pinion bearing in my rear differential. Replaced the 3rd member today and the noise is gone and my truck is much, much quieter. When I got the old one out, spinning the pinion flange I could tell the preload was way light.

Putting the swapped 3rd in, I read the wrong torque specs and snapped off one of the 10 studs that hold the housing on. I got it out, and then i put it back in a bit as a plug when I was refilling the gear oil and realized that oil would leak from the hole.

So I'm short a bolt.

It seems to still be leaking just a tad, from where the differential housing and axle housing meet. I used just a tiny bit of RTV when I put the swapped 3rd in, and it still had its papery gasket on it (torn a little though). Going to get a new stud/washer/nut this week. Hopefully that fixes the leaking. I do NOT want to take the housing off for a third time.